Aerospace turbofan engines are designed to withstand in-service loads arising from a variety of operational conditions. One such condition is the sudden release of a fan blade, which is termed a Fan Blade Off event. Such an event causes high impact loads which in turn must be accommodated by the pylon-engine-nacelle structure. The resulting force may be estimated from the following equation.Ffbo=mω2r 
Assuming a fan blade weight of 10 kg, a nominal rotational speed of 50 revolutions per second and a radius of 1 m, the fan blade off force becomes:Ffbo=10*(50*2π)2*1Ffbo=9.86*105N 
It is known to incorporate a mechanical fuse element into the rotational part of the engine on medium to large turbofan engines to dissipate the significant force resulting from a Fan Blade Off event.
This mechanical fuse element is incorporated into the front bearing arrangement as shown in FIGS. 1A and 1B. When the fuse element breaks, the support stiffness drops significantly which causes a phase inversion in the equations of motion for the shaft assembly. This results in the out of balance forces generated by the release of the fan blade being reacted primarily by the inertia of the fan assembly, which requires that the rotor orbit (e) must be large.
Together with the inertia reaction, a gyroscopic moment is generated by the precession of the shaft assembly axis (angle α).
In this arrangement, the gyroscopic moment makes a smaller contribution to opposing the out of balance forces generated by the Fan Blade Off event.
Statements of Disclosure
According to a first aspect of the present disclosure there is provided a shaft assembly, the shaft assembly comprising:                a first shaft portion, connected to a second shaft portion by a flexible coupling,        the first shaft portion being further connected to the second shaft portion by a frangible coupling, the frangible coupling being configured to fail if a bending load across the frangible coupling exceeds a predetermined maximum bending load,        the flexible coupling allowing an angular misalignment between the first and second shaft portions not exceeding a maximum angular misalignment.        
The shaft assembly of the disclosure moves the mechanical fuse element into the shaft to release a rotational degree of freedom, in contrast to the prior art arrangement of releasing a radial degree of freedom.
Following the loss of a fan blade and the corresponding failure of the frangible coupling, the radial supporting stiffness of the fan shaft assembly becomes very low. This results in the natural frequencies of the fan shaft assembly (that in turn are governed by the relationship between the dynamic stiffness and the dynamic inertia forces) likewise being very low. In this situation, the fan shaft assembly is said to be operating ‘super critically’, which means that its rotational speed is greater than its resonant speed.
When the fan shaft assembly is operating in this ‘super critical’ region, the inertia force is much larger than the stiffness force and so the fan shaft assembly will rotate about its centre of inertia which is significantly radially offset by the loss of the fan blade.
The gyroscopic couple generated by the angular precession of the fan shaft assembly introduces a moment at 90° to the precession which acts similarly to a stiffness. With the frangible coupling being close to the fan at the end of the fan shaft assembly, the size of the gyroscopic couple that is generated becomes significantly larger than the gyroscopic couple generated with the prior art fusing technique. Consequently, the magnitude of this gyroscopic couple is sufficiently large as to be able to provide support to the fan portion of the fan shaft assembly.
This means that the gyroscopic forces resulting from the failure of the frangible coupling can be tailored by adjusting by the axial distance of the frangible coupling hinge point from the centre of inertia of the fan portion of the fan shaft assembly. The gyroscopic forces can therefore be used to control the orbit size and the natural frequencies of the fan portion. The natural frequencies of the system are very difficult to adjust by traditional design but are critical on run down following fan blade off to avoid other natural frequencies (i.e. the engine structure on the wing) or operational speeds (i.e. ‘windmilling’ operation).
By allowing an axial misalignment between the first and second shaft portions, a significantly greater precession angle is provided. This in turn results in a much greater gyroscopic moment that resists the out of balance forces generated by the Fan Blade Off event.
Optionally, the shaft assembly further comprises an axial load carrying element, and wherein the axial load carrying element provides an axial connection between the first shaft portion and the second shaft portion.
The axial load carrying element provides increased axial strength in the shaft assembly. This assists in ensuring that the frangible coupling does not fail under axial thrust loading. This in turn ensures that the frangible coupling fails only under the action of a bending moment load that exceeds a pre-determined value.
Optionally, the first and second shaft portions are arranged in axial series and the frangible coupling extends axially between the first and second shaft portions.
In one arrangement, the first and second shaft portions are connected in axial series with the frangible coupling extending axially between corresponding opposing ends of the first and second shaft portions. This arrangement is simple and cost effective to produce.
Optionally, wherein the first and second shaft portions are arranged concentrically and the frangible coupling extends radially between the first and second shaft portions.
In another arrangement, the first shaft portion is concentric with the second shaft portion with the frangible coupling extending in a radial plane between the first and second shaft portions.
This allows for a more compact shaft assembly because the axial length of the shaft assembly can be reduced.
Optionally, the flexible coupling comprises an axial load carrying element.
In another arrangement, the flexible coupling takes the form of a ball and socket joint that combines angular articulation with the ability to withstand axial loading. Such an arrangement may be more compact and weight efficient but is likely to be more costly than an arrangement with discrete flexible coupling and axial load carrying element.
Optionally, the flexible coupling is a membrane coupling.
A membrane coupling provides articulations through bending of the radially extending membranes while also providing for axial alignment. Such an arrangement is simple and cost effective.
The membrane coupling also provides an aligning stiffness which opposes the misalignment of the first and second shaft portions. This stiffness would act to increase natural frequencies and support the fan portion of the fan shaft assembly against gravity at low rotational speeds, such as during ‘windmilling’.
Optionally, the flexible coupling is a constant velocity joint.
A constant velocity joint (such as a Rzeppa joint) allows rotation of the shaft assembly when the first and second shaft portions are misaligned without consequent rotational speed variation.
Optionally, the constant velocity joint incorporates a ratchet assembly, the ratchet assembly allowing angular movement of the constant velocity joint in only one angular direction.
Following failure of the frangible coupling, the ratchet assembly permits angular movement of the constant velocity joint but prevents this movement from being reversed. This ensures that once the fan portion of the fan shaft assembly has moved in an angular sense it is constrained by the ratchet assembly to stay in this orientation while the rotational speed of the shaft assembly reduces to ‘windmilling’.
Optionally, the shaft assembly further comprises a bump stop, wherein the bump stop limits the angular axial misalignment between the first and second shaft portions.
Providing a limit to the angular misalignment between the first and second shaft portions helps to prevent egress of the fan assembly from the engine nacelle.
Optionally, the bump stop comprises one or more resilient elements.
The use of resilient elements reduces the magnitude of reaction loads arising from the angular misalignment between the first and second shaft portions reaching its maximum extent.
Optionally, the frangible coupling is concentric with the flexible coupling.
In another arrangement, the frangible coupling is positioned concentrically with and radially inwardly of the flexible coupling. This makes the shaft assembly more compact and weight efficient.
According to a second aspect of the present disclosure there is provided a gas turbine engine comprising a shaft assembly as claimed in the first aspect.
Other aspects of the disclosure provide devices, methods and systems which include and/or implement some or all of the actions described herein. The illustrative aspects of the disclosure are designed to solve one or more of the problems herein described and/or one or more other problems not discussed.
It is noted that the drawings may not be to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings.